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1.
Toughness-strength relations in the overaged 7449 al-based alloy 总被引:6,自引:0,他引:6
This article examines the relationship between plane strain fracture toughness, K
Ic
, the tensile properties, and the microstructure of the overaged 7449 aluminum-plate alloy, and compares them to the 7150
alloy. The 7449 alloy has a higher content of η′/η precipitates; and, the 7150 alloy contains a greater amount of coarse intermetallic particles, as it contains an appreciable
amount of coarse S phase (Al2CuMg), which is largely absent in the 7449 alloy. The toughness of the alloys shows an increase on overaging, and the 7449
alloy shows a reasonably linear toughness—yield strength relation on extended overaging. Several mechanisms of failure occur:
coarse voiding at intermetallics and a combined intergranular/transgranular shear fracture mode, with the former becoming
more important as overaging progresses. Drawbacks of existing models for toughness are discussed, and a new model for plane
strain fracture toughness, based on the microstructurally dependent work-hardening factor, K
A
, introduced in Ashby’s theory of work hardening, is developed. This model predicts a linear relation between K
Ic
and K
A
/0.85
/σ
ys
/0.35
, where σ
ys
is the yield strength, which is consistent with the experimental data. 相似文献
2.
An investigation of the plastic fracture of AISI 4340 and 18 Nickel-200 grade maraging steels 总被引:1,自引:0,他引:1
The mechanisms of plastic fracture (dimpled rupture) in high-purity and commercial 18 Ni, 200 grade maraging steels and quenched
and tempered AISI 4340 steels have been studied. Plastic fracture takes place in the maraging alloys through void initiation
by fracture of titanium carbo-nitride inclusions and the growth of these voids until impingement results in coalescence and
final fracture. The fracture of AISI 4340 steel at a yield strength of 200 ksi (1378 MN/mm2) occurs by nucleation and subsequent growth of voids formed by fracture of the interface between manganese sulfide inclusions
and the matrix. The growth of these inclusion-nucleated voids is interrupted long before coalescence by impingement, by the
formation of void sheets which connect neighboring sulfide-nucleated voids. These sheets are composed of small voids nucleated
by the cementite precipitates in the quenched and tempered structures. The sizes of non-metallic inclusions are an important
aspect of the fracture resistance of these alloys since the investigation demonstrates that void nuclea-tion occurs more readily
at the larger inclusions and that void growth also proceeds more rapidly from the larger inclusions. Using both notched and
smooth round tensile specimens, it was demonstrated that the level of tensile stress triaxiality does not effect the void
nu-cleation process in these alloys but that increased levels of triaxial tension do result in greatly increased rates of
void growth and a concomitant reduction in the resistance to plastic fracture. 相似文献
3.
Yen-Hung Tan Sheng-Long Lee Yu-Lom Lin 《Metallurgical and Materials Transactions A》1995,26(11):2937-2945
The effect of Be and Fe content on the plane strain fracture toughnessK
IC of aluminum-based A357 alloys is investigated. The fracture behavior of A357 alloys has been evaluated as a function of both
the magnitude and morphology of iron-bearing compounds and silicon particles. Addition of Be is beneficial for tensile properties
and fracture toughness in the case of alloys containing intermediate (0.07 pct) and higher (0.15 pct) Fe levels. On the other
hand, Be added to alloys containing the lower Fe (0.01 pct) level appears detrimental to tensile strength, but the quality
index, notch-yield ratio (NYR), and plane strain fracture toughness were improved. Fractographic analysis reveals that crack
extension of A357 alloys occurs mainly in an intergranular fracture mode. The fracture processes are initiated by void nucleation
at iron-bearing compounds or irregularly shaped eutectic silicon particles as a result of their cracking and decohesion from
the matrix. Then, void growth and coalescence result in growth of the main crack by shear-linkage-induced breakdown of submicronstrengthening
particles. The effect of Be on increasingK
IC is more apparent in the higher Fe alloys than in the lower Fe alloys. Superior toughness obtained by microstructural control
has also been achieved in the intermediate and higher Fe levels of Be-containing alloys, with values equal to those obtained
in alloys of lower Fe content. 相似文献
4.
Shingo Mineta Shigenobu Namba Takashi Yoneda Kyosuke Ueda Takayuki Narushima 《Metallurgical and Materials Transactions A》2010,41(8):2129-2138
The microstructures of as-cast and heat-treated biomedical Co-Cr-Mo (ASTM F75) alloys with four different carbon contents
were investigated. The as-cast alloys were solution treated at 1473 to 1548 K for 0 to 43.2 ks. The precipitates in the matrix
were electrolytically extracted from the as-cast and heat-treated alloys. An M23C6 type carbide and an intermetallic σ phase (Co(Cr,Mo)) were detected as precipitates in the as-cast Co-28Cr-6Mo-0.12C alloy;
an M23C6 type carbide, a σ phase, an η phase (M6C-M12C type carbide), and a π phase (M2T3X type carbide with a β-manganese structure) were detected in the as-cast Co-28Cr-6Mo-0.15C alloy; and an M23C6 type carbide and an η phase were detected in the as-cast Co-28Cr-6Mo-0.25C and Co-28Cr-6Mo-0.35C alloys. After solution treatment, complete precipitate
dissolution occurred in all four alloys. Under incomplete precipitate dissolution conditions, the phase and shape of precipitates
depended on the heat-treatment conditions and the carbon content in the alloys. The π phase was detected in the alloys with carbon contents of 0.15, 0.25, and 0.35 mass pct after heat treatment at high temperature
such as 1548 K for a short holding time of less than 1.8 ks. The presence of the π phase in the Co-Cr-Mo alloys has been revealed in this study for the first time. 相似文献
5.
The influence of Mn content on the ductile-brittle transition in 16 to 36 wt pct Mn steels was investigated and interpreted
in light of the evolving microstructure. It was found that when hcp ε martensite is present in the as-quenched condition or
forms during deformation, it lowers the toughness. In 25Mn steel, the stress concentrations at e plate intersections result
in the formation of planar void sheets along the {111}γ planes. The deformation-induced α’ martensite in 16 to 20 pct Mn alloys enhances the toughness, but leads to a ductile-to-brittle
transition at low temperatures that is due to the intrusion of an intergranular fracture mode. Binary alloys with greater
than 31 pct Mn also fracture in an intergranular mode at 77 K although the impact energy remains quite high. Auger spectroscopy
of the fracture surfaces shows no evidence of significant impurity segregation, which suggests the importance of slip heterogeneity
in controlling intergranular fracture in these alloys. 相似文献
6.
Jun Kameda 《Metallurgical and Materials Transactions A》1981,12(12):2039-2048
A study has been made of the influence of intergranular solute segregation on fracture toughness K1c in a series of Ni-Cr steels individually doped with Sb, Sn, and P. By means of toughness measurements in steels having two
different intergranular Sb distributions, of measurements of acoustic emissions and of scanning electron micrographs of a
load-interrupted and post-test-fatigued specimen, the values of K1c, computed from the “pop-in” load of the loadvs clip gauge displacement curves, are found to represent the formation of many patches of contiguous intergranular microcracks
ahead of the precrack. The present experiments demonstrate that in the early stage of solute segregation, K1c decreases more substantially than does the strength of grain boundaries σ* (measured in the notched bar tests), although
the embrittlement effects of metalloid elements are the same order for both K1c and σ*. A proposed model for the stress-gradient-control of brittle fracture supports the finding that the measurements of
K1c give a distorted view of the progress of intergranular embrittlement. 相似文献
7.
Two microscopic ductile fracture processes have been established in a fracture tough superalloy, Inconel 718, aged to five
strength levels. At yield strengths less than 800 MPa, the mechanism is a slow tearing process within large pockets of inhomogeneous
carbides and nitrides, giving rise to plane strain fracture toughness (K
IC)values greater than 120 MPa-m1/2. At yield strengths greater than 900 MPa, the mechanism involves fracture initiation at carbides and nitrides followed by
off crack plane void sheet growth nucleated at the Laves (σ) phases. Here, the fracture toughness drops to about 80 MPa-m1/2. A Mode I normal strain growth model for low yield strength conditions and a shear strain void sheet model for high yield
strength ones are shown to model KIC data obtained from a J-integral evaluation of compact tension results. 相似文献
8.
本文对几种工业常用的时效硬化铝合金和新研制的Al-Li系合金8090的拉伸性能和断口进行了研究和比较。结果表明,在峰值时效条件下,Al-Li系合金显示出较高的沿晶开裂倾向,而其它常用的时效硬化铝合金,无论拉伸性能如何,均发生韧窝型开裂。Al-Li系合金高的沿晶开裂倾向是由该合金较高的时效沿晶析出现象所决定的。 相似文献
9.
Lisa M. Young George A. Young John R. Scully Richard P. Gangloff 《Metallurgical and Materials Transactions A》1995,26(5):1257-1271
The aqueous environment-assisted cracking (EAC) behavior of two peak-aged beta-titanium alloys was characterized with a fracture
mechanics method. Beta-21S is susceptible to EAC under rising load in neutral 3.5 pct NaCl at 25 °C and −600 mVSCE, as indicated by a reduced threshold for subcritical crack growth (K
TH
), an average crack growth rate of up to 10 μms, and intergranular fracture compared to microvoid rupture in air. In contrast,
the initiation fracture toughness (K
ICi
) of Ti-15-3 in moist air is lower than that of Beta-21S at similar high σYS (1300 MPa) but is not degraded by chloride, and cracking is by transgranular microvoid formation. The intergranular EAC susceptibility
of Beta-21S correlates with both α-colonies precipitated at β grain boundaries and intense slip localization; however, the
causal factor is not defined. Data suggest that both features, and EAC, are promoted by prolonged solution treatment at high
temperature. In a hydrogen environment embrittlement (HEE) scenario, crack-tip H could be transported by planar slip bands
to strongly binding trap sites and stress/strain concentrations at α colony or β grain boundaries. The EAC in Beta-21S is
eliminated by cathodic polarization (to −1000 mVSCE), as well as by static loading for times that otherwise produce rising-load EAC. These beneficial effects could relate to
reduced H production at the occluded crack tip during cathodic polarization and to increased crack-tip passive film stability
or reduced dislocation transport during deformation at slow crack-tip strain rates. High-strength β-titanium alloys are resistant,
but not intrinsically immune to chloride EAC, with processing condition possibly governing fracture.
Formerly Graduate Research Associate, University of Virginia
Formerly Graduate Research Associate, University of Virginia 相似文献
10.
Metallurgical factors affecting fracture toughness of aluminum alloys 总被引:13,自引:0,他引:13
Crack extension in commercial aluminum alloys proceeds by the “ductile” or fibrous mode. The process involves the large, ~1
μm to ~10μm, Fe-, Si-, and Cu-bearing inclusions which break easily, and the growth of voids at the cracked particles. The
linking-up of the voids is accomplished by the rupture of the intervening ligaments, and this is affected by the fine, ~0.01μm
precipitate particles that strengthen the matrix. The ~0.1μm Cr-, Mn-, and Zr-rich intermediate particles are more resistant
to cracking and may enter the process in the linking-up stage. The fracture toughness of aluminum alloys therefore depends
on a) the extent of the heavily strained region ahead of the crack tip, which is a function of the yield strength arad modulus,
b) the size of the ligaments which is related tof
c, the volume fraction of cracked particles, and c) the work of rupturing the ligaments. An approximate analysis predicts KIc varies asf
c-1/6, and this is in agreement with measurements on alloys with comparable yield strength levels. Studies in which the aging
conditions are altered for the samef
cshow that the toughness decreases with increasing yield strength level. This degradation in toughness is related to the localization
of plastic deformation. The tendency for localization is illustrated with the help of “plane strain” tension and bend specimens
whose behavior is related to the toughness. Measurements of the strain distribution on the microscale show that slip is relatively
uniformly distributed in a 7000-type alloy with low inclusion and particle content when the material is in the as-quenched
and overaged conditions. In contrast the distribution is highly nonuniform in the peak aged condition where slip is concentrated
in widely spaced superbands involving coarse slip bands with large offsets that crack prematurely. The connection between
the tendency for slip localization and the fine precipitate particles which strengthen the matrix remains to be established.
In overaged alloys grain boundary ruptures occur within the superbands. The amount of intergranular failure increases with
grain size and is accompanied by a loss of fracture toughness.
This paper is based on an invited presentation made at a symposium on “Advances in the Physical Metallurgy of Aluminum Alloys”
held at the Spring Meetings of TMS-IMD in Philadelphia, Pennsylvania, on May 29 to June 1, 1973. The symposium was co-sponsored
by the Physical Metallurgy Committee and the Non-Ferrous Metals Committee of TMS-IMD 相似文献
11.
The effect of the retrogression and reaging treatments (RRA) on the microstructure of Al-7075 in the T651 temper, both in
the matrix and on grain boundaries, was studied using transmission electron microscopy. The processes occurring in the matrix
during the retrogression treatment are principally the dissolution of small particles of the η’ transition phase, transformation
to η of the larger particles of η’, coarsening of the three commonly observed variants of the η phase precipitates (η1, η2, and η4), and precipitation of new η phase particles, particularly the η1 variant. The main process occurring during the reaging treatment is either growth of partially dissolved η’ particles or
precipitation of the η’ phase. These lead to a microstructure containing many fine η’ precipitates and some larger η1 and η2 particles with a smaller amount of coarse η4 particles, resulting in a broad particle size distribution. The high strength of the 7075 alloy in the RRA temper is believed
to arise from the relatively high overall concentration of particles in this dispersion. The retrogression treatment produces
rapid initial coarsening of the grain boundary particles, which are primarily η phase precipitates, resulting in an increase
in their volume per unit grain boundary area,V
A
. The beneficial effect of the RRA treatment on the susceptibility of 7075-T651 to SCC is believed to be due, at least partially,
to the increased value ofV
A produced by the RRA treatment.
Formerly Visiting Assistant Research Engineer in the Department of Materials Science and Engineering, University of California,
Los Angeles, CA 相似文献
12.
The intermediate-temperature embrittlement of a hot-rolled ferritic spheroidal graphite cast iron was studied with the consideration
that triaxial stress is induced by the ellipsoidal graphite particles of three unequal axial radii. The graphite shape was
changed by various rolling reductions, and the tensile tests were performed at 673 K. The results show that the elongation
and flow stress are independent of rolling reduction, and intergranular fracture occurs in all specimens. In the plasticity
analysis, the triaxiality ratio (σ
m
σeq) at a point in the ferrite matrix center can be expressed in terms of graphite shape ratio (b
d
) and graphite interparticle spacing (2a
d
) as σ
m
/σeq = 1/3 +a
d
/(2b
d
) where σ
m
is the hydrostatic tensile stress and σeq is the equivalent stress. Accordingly, the average triaxiality ratio in the matrix center region is independent of rolling
reduction and greater than one, a result that is consistent with the fact that the elongation is about constant, and all specimens
undergo intergranular fracture. Moreover, the rolling reduction independent flow-stress behavior can be rationalized by the
analytical result that the average σ
m
/σeq is unchanged with rolling reduction, where σ
z
is the internal stress along the tensile direction. 相似文献
13.
Douglas M. Symons Anthony W. Thompson 《Metallurgical and Materials Transactions A》1997,28(3):817-823
The effect of hydrogen on the fracture toughness behavior of a nickel-base superalloy, Alloy X-750, in the solutionized and
aged condition was investigated. Notched bend specimens were tested to determine if the fracture process was stress or strain
controlled. The fracture was observed to initiate at a distance between the location of maximum stress and maximum strain,
suggesting that fracture required both a critical stress and strain. The effect of hydrogen was further investigated and modeled
using fracture toughness testing and fractographic examination. The fracture toughness of the non-charged specimen was 147
MPa√m. Charging with hydrogen decreased the fracture toughness, K
Ic
, to 52 MPa√m at a rapid loading rate and further decreased the toughness to 42 MPa√m for a slow loading rate. This is consistent
with the rate-limiting step for the embrittlement process being hydrogen diffusion. The fracture morphology for the hydrogen-charged
specimens was intergranular ductile dimple, while the fracture morphology of noncharged specimens was a mixture of large transgranular
dimples and fine intergranular dimples. The intergranular failure mechanism in Alloy X-750 was a microvoid initiation process
at grain boundary carbides followed by void growth and coalescence. One role of hydrogen was to reduce the void initiation
strain for the fine intergranular carbides. Hydrogen may have also increased the rate of void growth. The conditions ahead
of a crack satisfy the critical stress criterion at a much lower applied stress intensity factor than for the critical fracture
strain criterion. A model based on a critical fracture strain criterion is shown to predict the fracture behavior. 相似文献
14.
Mechanism of detrimental effects of carbon content on cleavage fracture toughness of low-alloy steel
The variation in fracture toughness of low-alloy base steels and weld steels with carbon contents of 0.08 and 0.21 wt pct
was investigated using notched and precracked specimens tested at low temperatures. The attention is focused on the mechanism
associated with detrimental effects on cleavage fracture toughness resulting from increasing carbon content. Analyses reveal
that, in the case of constant ferrite grain sizes with increasing carbon content, the yield stress σ
y
increases and the local fracture stress σ
f
remains constant for notched specimens. For precracked specimens, the σ
y
increases, whereas the σ
f
decreases. In both cases, the ratio σ
f
/σ
y
decreases; this ratio is one of the principal factors inducing the deterioration in the cleavage fracture toughness of the
higher carbon steels. Analyses also reveal that the critical strain for initiating a crack nucleus, which decreases with increasing
carbon content and impurity elements, appears to be another principal factor that has a negative effect on the fracture toughness
in both notched and precracked specimens. The results of the fracture toughness measured for weld metal with various grain
sizes further support the predominant effect of grain size on the toughness of notched specimens. 相似文献
15.
The results of a recent study of the effects of ternary alloying with Ti on the fatigue and fracture behavior of a new class
of forged damage-tolerant niobium aluminide (Nb3Al-xTi) intermetallics are presented in this article. The alloys studied have the following nominal compositions: Nb-15Al-10Ti
(10Ti alloy), Nb-15Al-25Ti (25Ti alloy), and Nb-15Al-40Ti (40Ti alloy). All compositions are quoted in atomic percentages
unless stated otherwise. The 10Ti and 25Ti alloys exhibit fracture toughness levels between 10 and 20 MPa√m at room temperature.
Fracture in these alloys occurs by brittle cleavage fracture modes. In contrast, a ductile dimpled fracture mode is observed
at room-temperature for the alloy containing 40 at. pct Ti. The 40Ti alloy also exhibits exceptional combinations of room-temperature
strength (695 to 904 MPa), ductility (4 to 30 pct), fracture toughness (40 to 100 MPa√m), and fatigue crack growth resistance
(comparable to Ti-6Al-4V, monolithic Nb, and inconnel 718). The implications of the results are discussed for potential structural
applications of the 40Ti alloy in the intermediate-temperature (∼700 °C to 750 °C) regime. 相似文献
16.
W. O. Soboyejo C. Mercer K. Lou S. Heath 《Metallurgical and Materials Transactions A》1995,26(9):2275-2291
The fatigue and fracture mechanisms in Ti-48Al-xMn (x = 1.4 to 2.0 at. pct) gamma-based titanium aluminide alloys are elucidated.
Unlike most gamma alloys, which fail predominantly by transgranular fracture at room temperature, fracture in ternary Ti-48Al-xMn
alloys is shown to occur mainly by intergranular failure. The incidence of intergranular failure increased with increasing
annealing duration and temperature. Intergranular fracture is shown to occur as a result of the segregation of Mn to equiaxed
and interlamellar boundaries. Annealing either above or below the eutectoid temperature results in the precipitation of α2 particles. The reduction in the strength and toughness of ternary Mn-containing alloys is attributed to the combined effects
of segregation and α2 precipitation. A micromechanics framework is presented for the assessment of twin toughening mechanisms under monotonie and
cyclic loading.
Formerly Staff Scientist with General Electric Research and Development, Schenectady, NY 12301
Formerly Undergraduate Student, Department of Materials Science and Engineering, The Ohio State University 相似文献
17.
J. J. Kruzic J. H. Schneibel R. O. Ritchie 《Metallurgical and Materials Transactions A》2005,36(9):2393-2402
Ambient- to elevated-temperature fracture and fatigue-crack growth results are presented for five Mo-Mo3Si-Mo5SiB2-containing α-Mo matrix (17 to 49 vol pct) alloys, which are compared to results for intermetallic-matrix alloys with similar compositions.
By increasing the α-Mo volume fraction, ductility, or microstructural coarseness, or by using a continuous α-Mo matrix, it was found that improved fracture and fatigue properties are achieved by promoting the active toughening mechanisms,
specifically crack trapping and crack bridging by the α-Mo phase. Crack-initiation fracture toughness values increased from 5 to 12 MPa√m with increasing α-Mo content from 17 to 49 vol pct, and fracture toughness values rose with crack extension, ranging from 8.5 to 21 MPa√m at
ambient temperatures. Fatigue thresholds benefited similarly from more α-Mo phase, and the fracture and fatigue resistance was improved for all alloys tested at 1300 °C, the latter effects being
attributed to improved ductility of the α-Mo phase at elevated temperatures. 相似文献
18.
Takahiro Sawaguchi Morio Sato Akira Ishida 《Metallurgical and Materials Transactions A》2004,35(1):111-119
The grain-size effect on shape-memory behavior of fine-grained Ti35.0Ni49.7Zr15.4 thin films was investigated by transmission electron microscopy. The films, with various grain sizes ranging from about 150
to about 400 nm, were prepared by heat treatment of amorphous films at the three different annealing temperatures of 773,
873, and 973 K, for three different annealing times of 5 minutes, 1 hour, and 10 hours. The film annealed at 773 K for 5 minutes
showed a nearly single phase of (Ti,Zr)Ni, while the films annealed at high annealing temperatures and/or long annealing times
showed λ
1 precipitates. For a high annealing temperature of 973 K, the critical yield stress (σ
c) was dominantly dependent on the grain size of the matrix, obeying the Hall-Petch equation. On the other hand, for a low
annealing temperature of 773 K, σ
c was dominantly dependent on the amount of λ
1 precipitates. The M
S temperature decreases almost linearly with increasing σ
c. The films showed sufficient fracture toughness, probably due to the nanometer scale of the grain size and the agglomerated
shape of λ
1 particles. 相似文献
19.
20.
A. B. Pandey B. S. Majumdar D. B. Miracle 《Metallurgical and Materials Transactions A》2000,31(13):921-936
Mechanical tests were performed on a powder-metallurgically processed 7093/SiC/15p discontinuously reinforced aluminum (DRA)
composite in different heat-treatment conditions, to determine the influence of matrix characteristics on the composite response.
The work-hardening exponent and the strain to failure varied inversely to the strength, similar to monolithic Al alloys, and
this dependence was independent of the dominant damage mode. The damage consisted of SiC particle cracks, interface and near-interface
debonds, and matrix rupture inside intense slip bands. Fracture surfaces revealed particle fracture-dominated damage for most
of the heat-treatment conditions, including an overaged (OA) condition that exhibited a combination of precipitates at the
interface and a precipitate-free zone (PFZ) in the immediate vicinity. In the highly OA conditions and in a 450°C as-rolled
condition, when the composite strength became less than 400 MPa, near-interface matrix rupture became dominant. A combination
of a relatively weak matrix and a weak zone around the particle likely contributed to this damage mode over that of particle
fracture. Fracture-toughness tests show that it is important to maintain a proper geometry and testing procedure to obtain
valid fracture-toughness data. Overaged microstructures did reveal a recovery of fracture toughness as compared to the peak-aged
(PA) condition, unlike the lack of toughness recovery reported earlier for a similar 7XXX (Al-Zn-Cu-Mg)—based DRA. The PA material exhibited extensive localization of damage and plasticity. The low toughness of
the DRA in this PA condition is explored in detail, using fractography and metallography. The damage and fracture micromechanisms
formed the basis for modeling the strength, elongation, toughness, and damage, which are described in Part II of this work.
This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October
11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials
Committee. 相似文献